DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



53 



pq 





pq 







P5 



53 -M 

 2 >> 



-k_ 



\ 



A 



If 



■A 



"K 



"fe 



ft A! 





Excellent illustrations of this feature are supplied by sections on the line of the 

 Cleveland dyke. Towards its south-eastern extremity this ►. i % % % % 



great band of igneous rock ascends from the low Triassic L ~~' " 



plain of the Tees into the high uplands of Cleveland. Its 

 course across the ridges and valleys there has been carefully 

 traced for the Geological Survey by Mr G. Barrow, who has 

 shown that over certain parts of its course it does not reach 

 the surface, but remains concealed under the Jurassic rocks, 

 which it never succeeded in penetrating. But that in places 

 it comes within a few feet of the soil is shown by the baked 

 shale at the surface, for the alteration which it has induced 

 on the surrounding rocks only extends a few feet from its 

 margin. These interruptions of continuity show how uneven 

 is the upper limit of the dyke. The characteristic porphyritic 

 rock may be observed running up one side of a hill to the 

 crest, but never reaching the surface on the other side. At 

 Cliff Eidge, for example, about three miles south-west of 

 Guisbrough, Mr Barrow has followed it up to the summit 

 on the west side ; but has found that on the east side it does 

 not pierce the shales, which there form the declivity. This 

 structure is represented in fig. 6. The vertical distance 

 between the summit to the left, where the dyke (b) disappears, 

 and the point to the right, where the Lias shale (a) of the 

 hill-side is concealed by drift (c), amounts to 250 feet, the 

 horizontal distance being a little more than 900 feet. But 

 as the shale when last seen at the foot of the slope is quite 

 unaltered, the dyke must there be still some little distance 

 beneath the surface, so that the vertical extension of this 

 upward tongue of the dyke must be more than 250 feet. Mr 

 Barrow, to whom I am indebted for these particulars, has 

 also drawn the accompanying section (fig. 7) along the course 

 of the dyke for a distance of nearly 1 1 miles eastward from 

 the locality represented in fig. 6. From this section, it will 

 be observed that in that space there are at least three tongues 

 or upward projections of the upper limit of the dyke. Several 

 additional examples of the same structure are to be seen 

 further east towards the last visible outcrop of the dyke. 



Another feature connected with the upward termination 

 of the dyke is well seen in some parts of the ground through 

 which the two foregoing sections are taken. Mr Barrow 

 informs me that at Ayton a level course has been driven 



VOL. XXXV. PART 2. 





C3 —3 





O 



o 



o 



a e| 









into the hill for mining 



H 



